The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 199 03 979.8, filed on Jan. 25, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a starch-based graft polymer, a process for its preparation, and the use thereof in printing inks and overprint varnishes.
2. Discussion of Background Information
Currently used binders for printing inks and overprint varnishes are usually based on polymer dispersions having a solids content of 40-50% by weight of styrene and its derivatives, which are used as copolymers with acrylic acid. This requires neutralization with high concentrations of ammonia or amines, resulting in undesirably high amounts of volatile components in the product. Synthetic starting materials are used for the preparation.
On the other hand, many starch-based products are known in the prior art. Natural or modified starch has many uses in the food, paper, textile, adhesive and other industries. Starch can be modified by physical and chemical action as well as by introducing foreign groups and by grafting reactions.
The invention provides an improved graft polymer by partial or complete employment of renewable raw materials or modifications thereof, which improved graft polymer is suitable for preparing polymer dispersions that have a negligible content of volatile components and can be used advantageously as binders in printing inks or overprint varnishes. At the same time, the property profile of the currently used binders is changed as little as possible. In particular, properties such as gloss, storage stability, compatibility, water resistance and processibility are at a comparable level.
The present invention relates to a graft polymer based on derivatized starch or derivatized starch product as the graft substrate. The starch or starch product is derivatized by one or more bifunctional monomers and is grafted, at sites of derivatization, with one ore more ethylene derivatives.
The present invention also relates to a process for preparing the graft polymers according to the present invention. The process includes providing an aqueous medium containing dissolved or dispersed starch or dissolved or dispersed starch product. This dissolved or dispersed starch or dissolved or dispersed starch product is subjected to derivatization with one or more bifunctional monomers to prepare a derivatized starch or a derivatized starch product. The derivatized starch or derivatized starch product then is graft-polymerized, at sites of derivatization, with one ore more ethylene derivatives.
The resulting graft polymer can be incorporated, for example, as a polymer dispersion, in a printing ink or an overprint varnish. Consequently, the present invention also relates to printing inks and overprint varnishes which include the graft polymer.
Finally, the present invention also relates to an aqueous dispersion containing the graft polymer. A corresponding aqueous dispersion is obtainable, for example, by the process of the present invention.
The process for preparing such a polymer dispersion proceeds in multiple steps. The first step (i) comprises providing (preparing) a solution or dispersion of starch or starch product and water.
Starch or starch products within the meaning of this invention are natural starches from various sources as further described below as well as modified starches such as partially degraded starches, intermediate starch products and the like. Therefore, in the following, the recited terms are used synonymously, that is, the following examples, although primarily relating to starch, also apply correspondingly to the starch products employed in the present invention.
The water-soluble or water-dispersible starch can be obtained from grains, such as corn, wheat, millet or rice as well as from tubers and roots, such as potatoes and tapioca, fruit, or legumes and other natural products. Starch products, such as dextrins or modified dextrins, can also be used advantageously. In particular, hydrolyzed starches may be used. For example, the so-called desiccated dextrins, such as yellow potato dextrin of high or average viscosity, octenyl-succinate-waxy-maize starch and/or oxidized waxy-maize starch may be used. Combinations of the aforesaid starches may also be used.
Preferred for use are one or more of the most water-soluble starches that are available, for example, in hydrolyzed form. Dissolution takes place generally in a suitable reactor provided with a heat source, a stirrer, a cooling device and a thermometer. Dissolution is accelerated by heating to about 85-95xc2x0 C. The dissolution step generally lasts about 1 to 2 hours. The degree of dissolution is monitored visually on samples withdrawn from the reactor at suitable time intervals. Sixty-micron coatings are made from these samples with a doctor blade and a glass plate. Monitoring is limited to the size and content of specks in the dried film. Dissolution or dispersion is considered complete only when the 60-micron film is almost speck-free and free of gel particles.
The second process step (ii) comprises making a derivative of the dissolved or dispersed starch with bifunctional monomers. The bifunctional monomers used for this purpose contain a vinyl group and a functional group that can be condensed with the free hydroxyl groups in the starch.
The bifunctional monomers N-methylolacrylamide, N-methylolmethacrylamide, hydroxyethyl methacrylate, hydroxypropyl methacrylate or mixtures thereof are preferred for use in the condensation reaction.
It is important to use specific catalyst systems and temperature ranges for successful condensation and later polymerization.
Examples of useful catalysts include aluminum chloride, aluminum zirconium acetate, ammonium chloride, ammonium phosphate, magnesium chloride, organic acids, such as lactic acid, citric acid, para-toluenesulfonic acid, sodium chlorate or sodium perchlorate, in combination with magnesium or zinc salts, zinc nitrate, or zinc perchlorate.
The second process step is conducted generally at a temperature of about 80 to about 100xc2x0 C., preferably about 90 to about 100xc2x0 C. and in particular, at about 90xc2x0 C. Reaction time is generally about 1 to 5, preferably about 2 to 4 and in particular, about 3 hours. For example, the condensation can be conducted at about 90xc2x00 C. and for a period of about 3 hours. However, varying reaction times may be required, depending on the production equipment and the reactor type and size. Adapting reaction times appropriately is a matter for the skilled artisan""s judgment.
The condensation reaction in the aqueous phase generally does not proceed to completion. For example, the reaction may be conducted to about 20% conversion of the bifunctional monomers. The remaining unreacted monomer will then be incorporated in the graft polymer during the subsequent radical polymerization and thus also contribute to the stability and the most favorable properties of the polymers.
The result of this process step can be followed analytically. The analysis may relate, for example, to the product of the condensation reaction involving the dissolved starch and N-methylolacrylamide. For this purpose, the condensate, for example, is precipitated with an about 7-fold quantity of ethanol and washed several times with a 50% ethanol solution. Then nitrogen is analyzed by the Kjeldahl method (according to DIN EN ISO 3188). After evaluation of the samples and blanks, the Kjeldahl nitrogen analysis generally shows that the condensation has proceeded to an extent of about 20%.
After the condensation reaction, the radical reaction with the ethylene derivative(s) can proceed as the third step (iii).
The resulting graft polymer is graft-polymerized with ethylene derivatives essentially through the starch sites derivatized by the bifunctional monomers. Therefore, the present invention also relates to graft polymers in which a small proportion of the graft polymerization has taken place on starch sites that have not become derivatized by the bifunctional monomers employed according to the present invention.
The following monomers are examples of monomers which may be used, alone or as mixtures, as ethylene derivatives: acrylic and methacrylic compounds, such as acrylic acid, methacrylic acid, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, N-methylolacrylamide, N-methylolmethacrylamide, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, isopropyl methacrylate, isobutyl acrylate, n-butyl acrylate, amyl acrylate, n-hexylmethacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, heptyl acrylate, dodecyl acrylate, octadecyl acrylate, octyl acrylate, n-butyl methacrylate, isobutyl methacrylate, decyl methacrylate, dodecyl methacrylate, octadecyl methacrylate, allyl acrylate, allyl methacrylate, 2-dimethylaminoethyl acrylate, 2-t-butylaminoethyl methacrylate, 2,3-epoxypropyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, ethyleneglycol dimethacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-ethyl-2-(hydroxymethyl)-propanediol-(1,3) trimethacrylate (trimethylolpropane triacrylate), glycidyl methacrylate, 2-ethoxyethyl methacrylate, 2-butoxymethyl methacrylate, furfuryl methacrylate, 2-trimethylammoniumethylenemethacrylate chloride, stearyl methacrylate, 2-methoxyethyl acrylate, 2-butoxyethyl acrylate, butanediol monoacrylate, butanediol diacrylate, hexanediol diacrylate, diethylaminoethyl acrylate, dimethylaminoneopentyl acrylate, ethyldiglycol acrylate, beta-phenoxyethyl acrylate, lauryl acrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dihydrodicyclopentadienyl acrylate, vinyl acrylate, triethyleneglycol diacrylate, tetraethyleneglycol diacrylate, tripropyleneglycol diacrylate, 3-methylpentanediol acrylate, ethyleneglycol dimethacrylate, butanediol dimethacrylate, neopentylglycol dimethacrylate, triethyleneglycol dimethacrylate, dibromopropyl acrylate, dimethylaminoethyl methacrylate; vinyl compounds, such as vinyl acetate, vinyl propionate, allyl acetate, diallyl succinate, divinyl adipate and vinyl ethyl-hexanoate, the vinyl ester of Versatic acid, N-vinyl-2-pyrrolidone, allyl alcohol, vinylsulfonic acid, maleic anhydride, maleic acid, fumaric acid, itaconic acid, sodium p-styrene sulfonate, dibutyl maleate, dibutyl fumarate, crotonic acid and hydrocarbons, such as ethylene, butadiene, styrene and alpha-methylstyrene.
Preferred ethylene derivatives include vinyl acetate, methyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl methacrylate, hydroxymethyl methacrylate, glycidyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, butadiene, styrene and maleic anhydride and mixtures thereof.
The graft polymerization with the ethylene derivatives is conducted preferably by a metering method. The metering time is generally between about 20 to about 180 minutes, preferably about 30 to about 150 minutes and in particular, about 30 to about 90 minutes. However, varying reaction times may be required, depending on the production equipment and the reactor type and size. Adapting reaction times appropriately is a matter for the skilled artisan""s judgment.
The reaction temperature should be in the range of about 60 to about 90xc2x0 C., preferably about 60 to about 80xc2x0 C. and in particular, about 70 to about 80xc2x0 C.
Examples of suitable catalysts include radical-forming initiators and redox systems. Exemplary radical-forming polymerization initiators that may be used include alkali and ammonium salts of peroxy acids, such as potassium, sodium and ammonium persulfate. Other polymerization initiators that may be used include hydrogen peroxide, perborates and azo compounds, such as azodiisobutyronitrile. Organic peroxides and hydroperoxides, such as benzoyl peroxide, t-butyl hydroperoxide, diisopropylbenzene hydroperoxide and t-butyl perbenzoate may also be used. Additionally, redox systems use activators, such as sodium hydrogen sulfite, sodium bisulfite, sodium formaldehyde sulfoxylate, ascorbic acid, n-dodecylmercaptan and n-butyl-3-mercaptopropionate, in combination with one or more initiators.
To attain the desired property profile, the starting materials are used preferably in specific ratios. Preferred ratios are in the range of about 20 to about 60% by weight, in particular, about 30 to about 50% by weight starch, about 1 to about 10% by weight, in particular, about 3 to about 7% by weight (for example, about 3 to about 5% by weight) bifunctional monomer, and about 30 to 79% by weight, in particular, about 43 to about 67% by weight ethylene derivative, in each case based on the solids content of the dispersion obtained. Too low a proportion of starch may result in polymer dispersions having a high content of agglomerate and very turbid polymer films having very low gloss. If the starch concentration becomes too high, gloss increases but the films become too hydrophilic, or the viscosity of the polymer dispersion increases, so that processing is no longer possible. Derivatization is necessary particularly in order to enable a subsequent grafting step. An adequate concentration of ethylene derivative results in properties, such as hydrophobicity and gloss, that are necessary for practical use.
After the termination of the reaction, the product is generally cooled to room temperature and adjusted to pH 8.5xc2x10.5 with technical, i.e., about 26% aqueous ammonia; less than about 1% by weight, relative to the polymer dispersion, of ammonia solution, is usually required. Additional conventional additives can be added to the binder, such as preservatives, neutralizers, defoamers, wetting agents and the like. Thus, for example, the polymer dispersion according to the present invention may be protected against microbial infestation by the addition of a commercial preservative based on isothiazolinone.
The solids content of the dispersions according to the present invention usually ranges from about 30 to about 70% by weight, preferably about 30 to about 60% by weight and in particular, about 40% by weight. The viscosity thereof usually is in the range of about 200 to about 1000 mPas, in particular, about 500 to about 600 mPas. The particle size of the graft polymer usually ranges from about 0.1 to about 0.6 microns, preferably about 0.2 to about 0.5 microns and in particular, about 0.28 to about 0.42 microns.
A clear advantage of the process according to the present invention is that it can be conducted without the addition of surfactants or emulsifiers.
Another advantage of the process according to the present invention is that steps (i) to (iii) can be conducted in immediate succession in the aqueous phase, preferably in a reaction device, without intermediate steps, such as separation, being required.
The resulting graft polymer can be incorporated, for example in the form of an aqueous polymer dispersion obtained as described above and by conventional techniques known to the skilled person, into printing inks, such as flexographic printing inks, or overprint varnishes.
The polymer dispersions according to the present invention are generally also outstanding for high viscosity stability, i.e., despite a high solids content, their viscosity does not change during an extended time period. The usual retrogradation in starch-containing products, which results in a viscosity increase or phase separation by creaming or sedimentation, is eliminated in these polymer dispersions by suitable selection of the starch products. The polymer dispersions according to the present invention are generally finely dispersed, and yield high gloss, speck-free coatings. They can be easily converted into films at room temperature with the usual film-forming auxiliary agents. As described above, neutralization with ammonia or amines is required to only a very slight extent, so that the proportion of volatile components is not to be considered critical. In general, the proportion of volatile components, excluding water, is below about 1% by weight, for example, about 0.1 to about 0.9% by weight, preferably below about 0.5% by weight.